Hydraulic cylinder, piston and rod combinations are being increasingly used in many fields, a principal one of which involves the moving and positioning of material and objects. As the various applications of hydraulic cylinder, piston and rod combinations have progressed, more stringent operational criteria are being encountered, and a need has developed to precisely, reliably and continuously sense the position of the piston and it's related displacement parameters, velocity and acceleration.
A promising approach developing in the art employs the correlation of resonant frequencies of electromagnetic waves in the cavity that is made up of the cylinder and the piston with the dimensions of the cavity. In one aspect of this approach, the wave performance is similar to that of an electrical transmission line with shorted ends, in that the resonance frequency of an electromagnetic standing wave correlates inversely with the cavity length.
In one application of the use of electromagnetic waves for piston position sensing, as shown in U.S. Pat. No. 4,588,953, electromagnetic waves are introduced into the cylinder between the closed end of the cylinder and the piston and wherein different counters are employed to increment the introduced frequency and to correlate the sensed frequency signal, when resonance is detected, with the position of the piston.
In U.S. Pat. No. 4,737,705, a coaxial resonant cavity with a central core is employed within which the electromagnetic waves are launched and propagated in the mode referred to as the transverse electromagnetic wave (TEM) mode. The cylinder on the rod side of the piston is one type of coaxial cavity.
In European Patent Office (EPO) published application number 0 199 224 published Oct. 29, 1986, in a hydraulic cylinder cavity between the closed end and the piston, a variable threshold is employed to distinguish particular frequencies out of a broad range of frequencies.
In U.S. Pat. No. 5,182,979, compensation, through equalization, is provided for differences in insertion losses as the linear extension of the piston and rod in the cylinder takes place.
In U.S. Pat. No. 5,325,063 identification of a fundamental resonance frequency is enhanced by the use of intercepts of the positive and negative slopes of a resonance signal envelope where the positive slope is at the lower frequency and the negative slope is at the higher frequency.
As progress in the art of electromagnetic wave piston position sensing in hydraulic cylinders continues, greater precision is being sought. Accommodation for variations in change of direction, differences between cylinders, temperature responsiveness, static vs dynamic operation and lifetime wear is needed.